OXIDATIVE DEHYDROGENATION OF ETHANE ON V-BASED AND CR-BASED PHOSPHATE CATALYSTS

Oxidative dehydrogenation of ethane to ethylene has been studied on V- and Cr-based phosphate catalysts at 550 degrees C, yielding almost exclusively ethylene and COx Zirconium hydrogenophosphates in their alpha or beta phase were either submitted to cationic exchange of protons by VO2+ and Cr3+ salts or impregnated by such salts and then compared to (VO)(2)P2O7 and CrPO4 pure phases. VO2+ and Cr3+ on the alpha Zr(HPO4)(2) . 2H(2)O phase were observed to be more selective toward ethylene than the same cations deposited or exchanged on beta Zr(HPO4)(2) and than the corresponding oxides V2O5 and Cr2O3 either pure (bulk) or deposited on supports as SiO2, ZrO2 or ZrP2O7. Moreover, (VO)(2)P2O7 (and to a lesser extent CrPO4) were observed to be even more selective toward ethylene at similar conversion levels. However, in terms of cation content per weight, the better yields of ethylene were obtained for alpha ZrCrP and Cr/alpha ZrP samples. The turnover number of ethane conversion per cation accessible was 0.26 min(-1) for both V and Cr cations exchanged or supported on an alpha ZrP phase. Ultraviolet (UV) and electron spin resonance (ESR) spectroscopic techniques, which are very sensitive to the oxidation state and environment symmetry of V and Cr cations, were used in addition to structural and morphological determination techniques such as X-ray diffraction (XRD), infrared (IR), BET, scanning electron microscopy (SEM), etc. Characterization was done after calcination in air at 500/550 degrees C and after catalytic testing at 550 degrees C. All data allowed us to conclude that catalytic features are related to V and Cr local arrangements (small clusters or chain arrangements) and to the counter anion, O2- or PO43-. The best catalyst for ethane oxidative dehydrogenation consisted of VO2+ and Cr3+ chains separated by PO43- anions, which are stronger bases in the sense of Pearson than O2- anions.